This invention relates to offshore structures, and more particularly to offshore structures adapted for supporting oil and gas exploration/production operations at sea.
Semi-submersible vessels represent one type of an offshore structure that is used for conducting operations at sea. Semi-submersible units are often used in deep waters, where conventional bottom-supported structures are difficult to install. A typical semi-submersible unit has a superstructure deck or decks supported by columns. The columns are supported by buoyant pontoons with ballast chambers that allow the vessel to be transported to a deployment site and be flooded with water to submerge the pontoons below the water surface. Once the pontoons are lowered to the desired depth, they provide stability and reduce vessel motion response to wind and water waves.
Some semi-submersible vessels are designed with a pair of parallel horizontal pontoons, which support vertically extending columns. Other designs provide for the use of the so-called “ring pontoon,” which is not necessarily circular. In most cases, the term “ring pontoon” is applied to square or rectangular pontoons that are defined by four interconnected pontoon portions. The object of the pontoons is to facilitate stability of a vessel in deep waters.
Various solutions have been offered to solve this problem. For instance U.S. Pat. No. 4,040,265 discloses a jack-up platform supported by legs that extend down and engage a mat structure resting on an ocean floor. The mat is a rectangularity-shaped structure having a plurality of chambers, where hydrocarbons may be stored. The end portions of the mat extend beyond the position of the support legs.
U.S. Pat. No. 4,907,912 illustrates another example of a jack-up rig using a submersible storage barge. During transit, the submersible storage barge-provides the flotation means for the rig hull. Once the unit reaches the drill site, the submersible barge is lowered together with the support legs until they rest upon a sea bottom. The submersible barge is at least flooded to stabilize its position on the sea bottom.
U.S. Pat. No. 6,374,764 discloses a self-floating offshore structure with one or more pontoons that are provided with lifting supports. The lifting supports, which may be rack-and-pinion gear driven jacks, telescoping hydraulic rams, system of cables and pulleys, are attached to a deck and move the deck relative to the pontoons. The pontoons may be lifted out of the water to provide an additional deck area.
While these designs work satisfactory under certain conditions, there exists a need for an offshore structure that is particularly adapted for development of small marginal fields that are located in deep waters and harsh environment.